This application relates generally to the detection of fatigue cracks in materials and, more particularly, to methods for detecting fatigue cracks in gas turbine components.
Vibratory, mechanical, and thermal stresses induced to an aircraft may cause fatigue cracks to develop in a variety of components. More specifically, low cycle fatigue (LCF) cracks may develop in any component that is subjected to cyclic stresses. Over time, continued operation with fatigue cracks may lead to component failures as the cracks propagate through the component. Detecting the cracks early in their growth may facilitate reducing component failures.
At least some known nondestructive evaluation (NDE) methods for inspecting components for fatigue cracks or other defects which could cause a failure of the engine or airframe, include for example, surface wave ultrasonic testing, eddy current testing, and fluorescent penetrant inspection (FPI). Generally, none of the known NDE methods are considered to be singularly capable of detecting LCF cracks with sufficient reliability, ease of application, and with reduced environmental, health, and safety (EHS) concerns. More specifically, unique geometries of some components may restrict the evaluation techniques that can be utilized, and at least some known methods are susceptible to errors and false indications from contaminants on the surface of the component being inspected, or contaminants within the cracks or defects. Additionally, at least some known NDE methods may inaccurately indicate a defect from surface roughness and other surface anomalies which do not result in component failure. In particular, craze cracking of coatings may cause multiple false indications using at least some known NDE methods.
To facilitate accurate more reliable results, without increasing EHS concerns, at least some components are inspected using infrared methods of NDE. Infrared NDE methods operate on the premise that all matter continuously absorbs and emits electromagnetic radiation. The continual motion of the charged particles within a material results in the emission of electromagnetic radiation. More specifically, the motion of the charged particles will increase with an increase in temperature and cause a corresponding increase in the continuous emission of radiation from the material. Cracks and defects typically absorb more radiation than other areas of the component, and as a result, the cracks also have a higher emissivity and radiance relative to the relatively flat and smoother surface areas surrounding the defect. However, such NDE techniques may not be able to distinguish between defects which could result in a failure and other minor surface anomalies which are not of great concern.